This invention relates to a centrifugal filtration method and a centrifugal filtration apparatus therefor.
In a chemical plant, a solid-liquid separation step of separating slurry which is untreated liquid into a liquid fraction contained in the slurry and crystal is often practiced in manufacturing of a product. Also, a cleaning step of cleaning the crystal with cleaning liquid and a drying step of drying cleaned crystal are generally conducted subsequent to the solid-liquid separation step.
An apparatus for such solid-liquid separation is generally classified into two types. One of them is a filter-type filtration apparatus which is adapted to squeeze or press slurry to feed it to a filter, to thereby separate the slurry into a liquid fraction and crystal. The other type is a centrifugal filtration apparatus which is generally constructed so as to feed slurry to a basket rotated at a high speed to separate the slurry into a liquid fraction and crystal by centrifugal force.
The filter-type filtration apparatus causes crystal to be pressed during filtration, so that the crystal tends to take the form of blocks or agglomerates. This results in the crystal being densified in structure. Thus, the filter-type filtration apparatus substantially fails to permit cleaning liquid to be uniformly distributed in the crystal by penetration in a cleaning step, to thereby keep the crystal from being effectively cleaned.
On the contrary, the centrifugal filtration apparatus permits suitable voids to be formed in a structure of crystal while eliminating such squeezing or pressing of crystal as described above, resulting in cleaning liquid uniformly entering crystal to a degree sufficient to ensure satisfactory cleaning of the crystal in a short period of time, so that the crystal may be provided with increased quality.
Also, the filter-type filtration apparatus has the above-described disadvantage of rendering a structure of crystal densified, so that much time is required for drying of the crystal; whereas the centrifugal filtration apparatus prevents densification of crystal, so that the drying may be accomplished in a relatively short period of time.
Thus, the centrifugal filtration apparatus ensures effective and uniform cleaning of crystal and reduces a period of time required for the drying, therefore, crystal obtained is substantially decreased in impurity content and provided with high quality.
The centrifugal filtration apparatus, as described above, exhibits significant advantages over the filter-type filtration apparatus, however, it encounters a problem of rendering recovery of crystal obtained troublesome or difficult because crystal is formed in a cage-type basket. In particular, when the basket is arranged in a casing of the closed type in order to prevent inclusion of impurities in the crystal, satisfactory recovery of crystal obtained is rendered substantially impossible.
It would be considered that recovery of crystal from the basket of the centrifugal filtration apparatus is conveniently carried out by inserting a suction pipe connected to a vacuum suction unit into the basket to recover the crystal by suction through the suction pipe. Certainly, the suction pipe permits crystal to be recovered to a degree even when the basket is arranged in a closed-type casing.
Nevertheless, mere insertion of the suction pipe into the basket is insufficient to efficiently recover substantially all crystal in the basket, leading to remaining of crystal in the basket.
In view of the foregoing disadvantage, the inventors proposed a centrifugal filtration apparatus as disclosed in Japanese Patent Publication No. 28553/1985 (60-28553) and Japanese Patent Publication No. 44982/1987 (62-44982). The apparatus proposed is so constructed that a basket is formed on a bottom wall thereof with circumferentially extending grooves in which crystal deposited on an inner periphery of the basket is collected by drop, resulting in crystal scraped off in the grooves by a scraping unit being discharged by suction using a suction pipe. The centrifugal filtration apparatus enhances recovery of crystal, however, it fails to collect all crystal in the grooves, leading to some remaining of crystal in the casket.
Further, in manufacturing of fine chemicals such as pharmaceutical preparations or the like, it is required to prevent inclusion of impurities in crystal to the utmost. For this purpose, it is desired to carry out steps extending from a solid-liquid separation step to a drying step in the same container. When a centrifugal filtration apparatus is used to this end, it is desired that crystal is dried in a basket after a solid-liquid separation step and a cleaning step. Thus, as seen in Japanese Patent Publication No. 20560/1995 (7-20560), it would be considered that the basket is fed with heat to dry crystal after completion of the cleaning step. Unfortunately, any conventional centrifugal filtration apparatus is not constructed so as to ensure that mere feed of heat to the basket permits crystal to be efficiently dried.